Question: A natural number is abundant if it is less than the sum of its proper divisors. What is the smallest abundant number?
Explanation: For any prime number, the sum of its proper divisors is equal to $1$, so a prime number cannot be an abundant number. Therefore, it suffices to check only composite numbers:

$\bullet$ For $4$, $1 + 2 < 4$,

$\bullet$ For $6$, $1 + 2 + 3 = 6$,

$\bullet$ For $8$, $1 + 2 + 4 < 8$,

$\bullet$ For $9$, $1 + 3 < 9$,

$\bullet$ For $10$, $1 + 2 + 5 < 10$,

$\bullet$ For $12$, $1 + 2 + 3 + 4 + 6 = 16 > 12$.

Thus, the answer is $\boxed{12}$.